Silicate pigments



2,739,073 SILICATE PIGMENTS Orlando Leonard Bertorelli, Borger, Tex., assignor' to J, M. Huber Corporation, New York, N. Y., a corporation of New Jersey No Drawing. Application September 6, 1950, Serial N0. 183,467

16 Claims; or. 106- 288) This invention relates to finely divided mineral pigments and, more particularly, to the production of'new white silicate pigments chemically analogous to zeolites but havare known to possess water softening properties but to v v have no usefulness as pigments. It also is well known that alkali silicates such as water glass can be reacted with aluminum sulfate to form gels which after drying. are granular in .nature and possess base exchange properties. Artificial zeolites of thattype contain sodium, aluminum and silicon oxides in various proportions which usually are controlled by the addition of sodium aluminatc during the reaction. In dried condition, they are hard granules having sharp edges and are exceedingly difficult to pulverize. Such artificial zeolites also lack essential characteristics of pigments. a v

It is an obect of the present invention to provide valuable new white pigments of the character described above, and another object is to provide a commercially practical method of producing such pigments. a

A further object is to provide ways of reacting alkali metal silicates and water soluble aluminum salts so as to produce new finely divided or powdery white substances of exceedingly small particle size, the particles of the reaction products being substantially all less than one micron in diameter and for the most part less than one-half micron in diameter.

More specific objects of this invention are to provide new silicate pigments which are especially useful in rubber compounds to increase the tear resistance of the rubber, and also to provide new white pigments especially useful in paper coatings as a means of imparting high brightness to coated paper.

It has now been discovered that silicate pigments fulfilling the above-mentioned and other important uses can be provided in the form of white powdery reaction prodnets of water soluble alkali metal silicates and water soluble salts of aluminum and strong acids. These products are compositions essentially of alkali metal, aluminum and When a dilute solution of an alkali silicate is mixed and reacted with a dilute solution of such an aluminum salt by pouring the two solutions directly together, the immediate result is the formation of a gel which may .hold'all the liquid present but still forms a solid mass. Although this mass can be broken up into small pieces by agitation, the pieces when dried are coarse and hard and are entirely unsuitable for pigment purposes. A gell formation of this nature occurs even when the two solutions are combined at concentrations as low as 0.5 molar.

According to this invention, it has been found that a distinctly diiferent effect yielding pigments of the type described can be obtained by'maintaining the reactant solutions at very low concentrations and under strong agitation during the process of mingling and reacting them together. In general, the concentration of the solutions at the point of reaction should average less than about 0.1 molar, depending somewhat upon the particular reactants employed, their purity and other'variables. In preferred embodiments'the reacting concentration probably is not more than about .01 molar.

The most satisfactory procedure for the purposes of this invention is to run dilute solutions of the reactants slowly into a diluting aqueous medium such as a body of Water, preferably at widely spaced locations therein, while continuously and vigorously stirring the reaction material so as to keep it in a fluid condition. The alkali silicate solution so used should be of about '2 molar or lower concentration, and the concentration of the aluminum salt solution should be about 1 molar or lower; While the volume of water which receives these solutions should be'atleast half their combined volume and preferably is greater than their combined volume. Although the reaction material'may thicken somewhat as the precipitation proceeds, at the end it is a fluid mass from which the precipitated solids are easily separated .by filtration, and the dilute stream of the silicate solution, having a concentrasilicon oxides, obtained in extremely small particle sizes,

as very fine precipitates, by suitably comingling and reacting together at very low concentrations dilute aqueous solutions of an alkali silicate and a water soluble alumi num salt such as aluminum sulfate, aluminum chloride, aluminum nitrate or ammonium alum. The precipitate pigment particles are substantially all less than one micron in diameter, and they average less than one-half micron in diameter; moreover, they are characterized by extraordinary brightness and by other qualities as hereinafter described which make them exceptionally valuable for uses of the nature indicated above.

resulting filter cake when dried is a soft white mass that readily disintegrates into exceedingly small pigment particles.

7 Another useable procedure is to combine slowly a very tion less than about 0.4 molar, with a similarly dilute stream of the aluminum salt solution while strongly agitating the reaction material, so that the water formed by reaction of the parts first combined soon serves as a diluting medium for completion of the precipitation at very low reactant concentrations. The desired pigment forming reaction occurs under highly alkaline conditions; so it is also practicable to place all of a dilute alkali silicate solution in a reaction container and then to run in very slowlya highly dilute solution of the aluminum salt while strongly agitating the material. This practice, however, is troublesome in requiring a more highly diluted inflow solution.

Accordingly, the preferred procedure for practicing this invention involves placing in a suitable vessel provided with a strong agitator an amount of water equal to at least half the volume of the solutions to be added and then, while continuously stirring the contents, introducing dilute solutions of the reactants in streams entering the water at widely spaced points. A high degree of dilution of each solution is thus assured before the solutions contact and react with each other. In a simple and effective arrangement, a propeller type agitator may be employed, and one solution may be added to the vortex which it creates While the other solution is added near the wall of the vessel. Alternatively, the two solutions may be introduced into the vessel at diiferent levels one of which is below the surface and th other at the surface on the agitated liquid.

The solutions should be combined at such relative inflow rates that the reaction material is maintained in Patented Mar. 2Q, 1956 v an l a ine condi i r g mo ofthe reaction and. at the end thereof. A fine powder is easily obtained in this way. In making a product to be used in rubber, the PH of the: reaction material should be held between about 8 and. 12,. preferably between about 9- and 1:1,until all the alkali silicate. has been added, and, then the addition of the aluminum salt solution preferably is. continued until? the. pH is reduced to between 8 and 9.5.

Upon completion of the reaction procedure. the precipiated pigment is. usually separated from the reaction liquid. by filtration, but other means of separation. such as centrifuging can be. used as well. It generally is desirable to wash the. freshly separated pigment withwater to, remove water soluble salts and the like, after which it,- may. be. dried in any suitable manner to obtain a friable mass which easily disintegrates. into. a fine powder. The drying preferably is. performed at elevated temperatures. When dried: at. l;l.0; C. the finished pigments usually contain about 9. to. 11% water, but this can bedriven Qfi by heating. them at considerably higher temperatures, for example, to. about 5.00 C.

Watch-soluble potassium silicates and sodium silicates can be. used similarly according to this invention, but the much less expensive sodium silicates naturally are preferred; They are effective inany composition in which the. ratio of SiQz. to.- alkali metal oxide is from 1 to about 3.5,. including the common alkali silicates ranging from. the meta silicate NazOtSiOa toordinary water glasswith a composition of about NazO.3 .36SiOz. Special pigment properties renderingthe products especially effective for certain uses may be promoted, however, by. selection of the silicate composition to be used.

Thus. it has: been found that exceedingly effective pigments for usein. rubber compounding, i. e., fine alkali aluminum silicate powders which greatly improve the tearresistance of rubber compounds containing them, can be obtained as substantially silica-free reaction prodnets. of an. alkali meta silicate such as NazOSiOzwith an aluminum salt such as aluminum sulfate. such products. a. dilute solution of sodium meta silicate preferably is reacted with. a. dilute solution of aluminum sulfate in the manner above described. Valuable rubber pigments of this type can alsobe obtained, however, by using. alkali silicates which contain from more than 1 to: about 3*.36.mola1'. equivalentsv of SiOz to each molar equivalent of alkali. metal oxide and, at the end of the described reaction. procedure, adding and vigorously stirring into the reaction. material an alkaline earth bydroxide solution. such as a solution of Ca(OH')2' in. an amount approximately. sufficient to react with the silica content inexcess of a ratio of 1 SiOz to l NazO. The pig ments thus obtained are believed to be composed of fine calciumtsilicate-coprecipitated with a fine reaction product ofi sodiumimeta silicate and aluminum sulfate. While the conversion. of: excesssilica into calcium silicate. or the like. is not necessary, this is important to the use of the pigments in rubber compounds because it overcomes a retarding effect on vulcanization which occurs when the pigments. contain. excess silica.

It. has. also. been. found, however, that the pigments prepared from. the alkalisilicates of higher silica content are excellently'suited for the preparation ofpapercoating colors. Products of this type are prepared in the described: manner from. silicates which contain SiO'a in a molarratio of from about 1.5 to about. 3.5 to their content'of NasO, such as disilicates. and. ordinary water glass. These pigments. exhibit a brightness in excess. of 9 0. as measured by a General Electric reflectance meter. Their particles not only are extremely small, being generally less than /2 micron in diameter, but they also have a distinct softness and other desired properties. in conjunction with their extreme brightness, which make them particularly valuable as a means for imparting high brightness. to paper coatings.

The. specific. gravity of the present products ranges To obtain M antfrom about 2.10 for pigments prepared from aluminum sulfate and sodium metal silicate to about 2.26 for pig ments prepared from aluminum sulfate and ordinary water glass. The pigments prepared from sodium meta silicate are completely soluble in a hot 20% solution of sodium hydrogen sulfate... While those prepared from silicates having a higher silica content are only partially soluble therein. It is believed that in all cases the pigment contains the same reaction product as that: which forms when using the meta silicate, but that this product is mixed with silica when silicates having a higher silica content are used. Small amounts of sulfate also are generally present, being trapped in the pigment.

The following examples are illustrative and indicate preferred ways of producing valuable finely divided pigments according to this invention:

Example 1 A 650-gallon kettle was provided with. a propeller type agitator having a blade 10 inches in diameter rotatable at. 900 R. P. M. A dilute alkali silicate solution. was prepared by dissolving 285 lbs. of sodium meta silicate (NazQSiOz) in 136 gals. of water, and. a dilute solution of aluminum sulfate (A12(SO4)3) was prepared by dissolving 187 lbs. thereof in 169 gals. of water. The kettle was charged with 1.85 gals. of water, and the agitator was started. Then the silicate solution was run in a thin stream directly into the vortex formed by the rotating propeller, and at the same time the aluminum sulfate solution was introduced, also in a thin stream, so as to strike'the surface of the vigorously agitated liquid near the wall of the kettle. Addition of both solutions continued'for about 15 minutes, during which the rates of inflowwere regulated so that the pH of the material stayedbetween 8" and 1-2. All the silicate solution having then been added, the addition of the aluminum sulfate solution was continued until the pH had dropped to 9.5. A total of 168 gels; of the alum solution was used. The; temperature during the reaction was 2632 C. Agitation of the reaction material was continued for an additional 15 minutes, and thenthe precipitate was separated by thefiltration and thoroughly washed with water. Theresulting filter cake was dried at C. to obtain a friable cake which disintegrated readily into a powder when squeezed. This cake. was passed once through a screen mill with the screen removed, in order to convert the agglomerated mass completely into a fine powder. Theyield was 623 lbs.

The material so produced was afine white powder of which all the particles were less than 0.5 micron in diameter, and about 97% of the particles were less than 0.1 micron in diameter. Its specific gravity was 2.10. Analysis of the product showed the following composition:

Per cent Ignition loss (water content) 10.20 SiGz 51.10 A1203 24.81 N'azO 13.03 S03 0.94

This pigment was tested in the followingrubber compounds:

After having been vulcanizedfor 8 minutes at 2.87" F.', this rubber compound. showed, an, abrasion loss of, 16 cc. and atear: resistance of; 443 lbs. Another rubber compound was prepared and. tested in the sarneway, except that the best grade of kaolin, a natural aluminum ,silicate, was substituted-inthe'. same volume for-thenew pigment of this invention. Theabrasion loss of this other compound was 29 cc. and its tear resistance was only 137 lbs. I

Example 2 This example was performed by use of the same'equipment as described in Example 1. The kettle was charged with 88 gallons of water at room temperature. A silicate solution was prepared by dissolving 272 lbs. of a sodium silicate having the composition Na20.1.91Si02 in 176 gallons of water, and an alum solution was prepared by dissolving 147 lbs. of aluminum sulfate (A12(S04)3) in 103 gallons of water. The two solutions were run during 12 minutes into the strongly agitated water, at rates regulated so that the pH of the material remained between 8 and 12. After adding all the silicate 'solution the final pH was adjusted to 8.7 by further addition of alum,

whereupon agitation alone was then continued for about minutes. The precipitate then present was filtered off, dried and pulverized as described in Example 1.

Substantially all of the particles of the resulting powder were smaller than 0.2 micron in diameter. The specific gravity of the pigment was 2.20. Its brightness as measured on a General Electric reflectance meter was 95.7. Analysis of the pigment showed the following composition:

- Per cent Ignition loss (water content) 11.19 SiOz 59.10 A1203 15.65 NazO 12.10 S03 1.92

A casein paper coating color was prepared to contain 22 parts of casein and 100 parts of the above pigment. This was applied to paper to form a coating which, though uncalendered, showed a brightness of 84.6. An uncalendered coated paper similarly prepared by use of a color containing 22 parts of casein and 100 parts of high grade water-washed kaolin showed a brightness of about 75.

In the pigment products of the foregoing examples, the ratio of NazO to A1203, after making allowance for the amount of sodium combined with S03, falls within the range of about .8 to 1.12 molar equivalents of NazO for each molar equivalent of A1203. This ratio may vary from as little as 0.80 to as high as 1.20, but it always remains within this relatively narrow range for all of the various pigments produced. The amount of SiO2 present in these pigments naturally varies more widely, being dependent upon the type of silicate employed and greatest in the use of ordinary water glass. The pigmentproduct prepared by the use of sodium metasilicate, as shown by calculations from the analysis given in Example 1, contains Si02 and Na2O in a molar ratio of about 4 to 1. When the silicate used contains substantially more silica than is present in the metasilicate, the molar ratio of SiOz to Na2O in the product is substantially greater than 4 to 1, as shown by calculations from the analysis given in Example 2. Example 3 A pigment was prepared exactly'as in Example 2, except that when the reaction was complete, a slurry of 59 lbs. of calcium hydroxide in 88 gallons of water was run quickly into the reaction material, and the mixture was then stirred for 30 minutes before filtration. The resultby-use of .sodium silicates containing, respectively, 1.4,

1.83., 2.34, 2.8, and 3.3 mols of SiO2 to l'mol of NasO. The pigments obtained in these practices possessed properties similar to those already described.

The new pigments produced according to this invention are particularly useful, as hereinbeforeindicated, in paper 1 coating colors and in the preparation of rubber compounds. They are useful not only in natural rubber compounds but also in compounds'of various synthetic rubbers or elastomers, such as chloroprene polymers, butadiene polymers, butene polymers, and the like. They also are valuable pigmentsfor use in paints, plastics, inks, protective coating compositions, and other similar materials wherein stable pigments of an exceedingly fine particle size and/or of high brightness properties are desired.

it will be understood that the foregoing detailed description and examples are illustrative only, for many variations and changes may be made in the reactants and in conditions of the process or the products without departing from the substance of the invention as herein disclosed and defined in the appended claims. 1

I claim:

1. As a new pigment, precipitated white powdery composition consisting essentially of oxides of sodium, aluminum and silicon substantially all the particles of which which are less than 0.5 micron in diameter, wherein the r 7 molar ratio of SiO; to Na20 is about 4 to 1 and the molar ratio of NazO to A120 is from .8 to 1.2 to 1, said composition having a specific gravity of from 2.10 to 2.26. v

3. As a new paper coating pigment, a precipitated white powdery composition consisting esentially of oxides of sodium, aluminum and silicon substantially all the particles of which are less than 0.5 micron in diameter,

and having a brightness in excess of as measured by a General Electric reflectance meter, wherein the molar ratio of Si02 to Na2O is substantially greater than 4 to 1 and the molar ratio of Na20 to A1203 is from .8 to 1.2 to 1, said composition having a specific gravity of from 2.10 to 2.26.

4. The process of producing a fine white sodium aluminum silicate pigment which comprises, while strongly agitating a body of a diluting aqueous medium, running slowly thereinto at spaced points therein a dilute aqueous solution of sodium silicate at not more than 2 molar concentration and a dilute aqueous solution of a salt of aluminum and a mineral acid, said salt solution being at not more than 1 molar concentration, said sodium silicate containing Si02 and Na2O in a ratio of from 1 to 3.5 to l and the combined solutions providing amounts of sodium and'aluminum equivalent to a NazOzAlzOa ratio of from .8 to 1.2 to 1, and keeping the resulting slurry alkaline during the reaction, thereby producing in a fluid reaction slurry very fine white precipitate particles composed principally of oxides of sodium, aluminum and silicon and substantially all less than 1 micron in diameter.

5. A process as described in claim 4, the initial volume of said diluting medium beingat least half the combined volume of said solutions.

6. A process as described in claim 4, the reaction slurry being held between pH 8 and 12 while combining said adilute aqueous solution of aluminum sulfate at not more than 1 molar concentration anda dilute aqueous solution 01- a sodium silicate-containing SiOz in a. molar ratio to NazO of from more than 1 toabout 3.36, the silicate solution being at not more than 2 molar concentration, thereby producing in said slurry at very fine precipitate composed essentially of oxides of sodium, aluminum and silicon, then incorporating intosaid slurry during strong agitation thereof an amount of alkaline earth hydroxide approximately sutficient to react" with the SiO2- content of said silicate solution in'exccss of an equimolecular ratioof SiQz to its NazO content, and then separating, drying and disintegrating the resulting precipitate to obtain discrete particles averaging less than 0.5 micron in diameter which enhance tear resistance in rub ber compounds.

8. The process of producing a white pigment especially valuable in paper coatings, which comprises commingling and reacting in a dilute aqueous slurry, while strongly agitating the slurry and keeping it alkaline, a dilute aqueous solution of aluminum sulfate at not more than 1 molar concentration and a dilute aqueous solution of a sodium silicate containing-SiOz in a molar ratio to NazO of about 125 to- 3 .5, the silicate solution being at not more than 2 molar concentration, thereby producing in said slurry a very fine precipitate composed essentially of oxides of sodium, aluminum and silicon, and then separating, dryingand disintegrating said precipitate to obtain discrete particles averaging less than 0.5 micron in diameter whichhave a specific gravity of about 2.10 to 2.26 and a brightness in excess of 90. as measured by a General Electric reflectance meter.

9. As a new pigment, a precipitated white powdery composition consisting essentially of oxides of sodium, aluminum and silicon, in which the molar ratio- NazOzAlzOa is from 0:8 to 1.211 and the molar ratio of 3120 to Nazi) is at least about 4 to 1, substantially all particles of which are less than 1 micron in diameter, said composition having a specific gravity of from 2.10 to 2.26.

10-. Asa new pigment, a precipitated white powdery composition consisting essentially of oxides of sodium, aluminum, silicon and an alkaline earth metal, in which the molar ratio NazOzAbOs is from 0.8 to 1.2:1 and the molar ratio SiOzINaeO is at least about 4 to l, substantially all particles of which are less than 1 micron in diameter, said composition having a specific gravity of from 2.10 to 2.26.

11. As a new pigment, a precipitated white powdery composition consisting essentially of oxides of sodium, aluminum, silicon and calcium, in which the molar ratio NazOzAlfiOs is from 0.8 to 1.211, the molar ratio of SiOzzNazO is more than 4 to l and the CaO content isapproximately equivalent stoichiometrically to the amount of SiO in excess of that satisfying a SiOz2NazO ratio of 4:1, substantially all particles of which are less than 1 micron in diameter, said composition having a specific gravity of from 2.10 to 2.26.

12. The process of producing a fine white pigment, which comprises continuously combining into a dilute aqueous slurry, while strongly agitating the slurry and keeping it alkaline, a stream of an aqueous sodium silicate solution at less than about 0.4 molar concentration and a stream of an aqueous solution of a salt of aluminum and a mineral acid at less than about 0.4 molar con centration, the silicate having a SiOzzNazO ratio of from 1 to 3.5 :1 and said streams providing amounts of sodium and aluminum equivalent to a NazOzAlzGa ratio of from 0.8 to 1.2:1, thereby producing in said slurry extremely fine white precipitate particles composed essentially of oxides of sodium, aluminum and silicon and substantially all-less than-1 micron; in diameter;

13-. The process ofproducing a fine white pigment, which comprises adding slowly into a: dilute aqueous slurrypwhilcastrougly agitating the slurry and keeping it alka'line; an. aqueousvsodium silicatev solution at. not more than 2 molar concentration. containing N320? and S162 in a ratiooffromtl to 36:1, and an. aqueous solution of a salt of aluminum and a mineral acid at not more than 1 molar concentratiomthe combined solutions Providing. amounts of. sodium and aluminum equivalent to aNazOzAlzOs ratio-of from 0.8 to 1.221, thereby producing in said. slurry extremely fine white precipitate particles composed essentially of oxides of sodium, aluminum and silicon and substantially all less than 1 micron in diameter.

14. The process of producing a fine white pigment, whichv comprises. adding slowly into a dilute aqueous slurry, while strongly agitating the slurry and keeping it alkaline, an aqueous sodium silicate solution at not more than 2 molar concentrationv containing NazO and sit): in a ratio of from more than 1 to 3.36:1, and an aqueous solution of a salt of aluminuntand a mineral acid at not more than 1 molar concentration, thecombinedsolutions providing amounts of sodium and aluminum equivalent to a. NazOzAlaOs ratio of from 0.8 to 1..2:1, and then adding into said, slurry an alkaline earth metal hydroxide to neutralize excess silica therein, thereby producing in said slurry extremely fineprecipitatc particles composed essentially. of oxides of sodium, aluminum, silicon and alakine earth metal and substantially all less than 1 micron: in diameter.

15-. The process which comprises commingling and reacting in a dilute aqueous medium, while keeping the medium alkaline and strongly agitating it so as to maintain the average reacting concentration at not more than about .01 molar, a dilute aqueous solution of an alkali metal silicate at not more than 2 molar concentration and a dilute aqueous solution of a salt of aluminum and a mineral-"acid, said saltsolution being at not more than 1 molarconcentration, said silicate containing SiOz and NazOin a ratio of frorn l to 3 .5 to l and the commingled solutionsproviding amounts of sodium and aluminum equivalent to a Na2O:Al2O3 ratio-of from .8 to 1.2 to 1, thereby producing insaid medium very fine white precipitate particles composed principally of alkali metal, aluminum andsilican'oxides and substantially all of which are less than 1- micron in diameter.

16 The process of producing" a pigment valuable in rubber compounds, which-comprises slowly commingling and reacting in a dilute aqueous medium, While strongly agitating themedium and keeping it alkaline, a dilute aqueous solution of sodium meta silicate at not more than 2 molar concentration and a dilute aqueous solution of aluminum sulfate at not more than 1 molar concentration, the comminglcd solutions providing amounts of sodiumand aluminum equivalent to a NazOzAlzOx ratio of from 0.8 to 1.2 to 1- and the pH of said medium being held between 8 and 12' throughout the reaction, thereby producing in said medium an extremely fine precipitate composed essentially of oxides-of sodium, aluminum and silicon and substantially all the particles of which are less than 0:5 micron in diameter, and separating, drying and disintegrating said precipitate toobtain a pigment which enhances tear resistance in rubber compounds.

ReferencesCitedin the; file of, this patent UNITED STATES PATENTS 

1. AS A NEW PIGMENT, PRECIPITATED WHITE POWDERY COMPOSITION CONSISTING ESSENTIALLY OF OXIDES OF SODIUM, ALUMINUM AND SILICON SUBSTANTIALLY ALL THE PARTICLES OF WHICH ARE LESS THAN 0.5 MICRON IN DIAMETER, WHEREIN THE MOLAR RATIO OF SIO2 TO NA2O IS AT LEAST ABOUT 4 TO 1 AND THE MOLAR RATIO OF NA2O TO AL2O3 IS FROM .8 TO 1.2 TO 1, SAID COMPOSITION HAVING A SPECIFIC GRAVITY OF FROM 2.10 TO 2.26. 